1. Field of the Invention
This invention relates to a communications adapter, and more particularly to such an adapter which can be used both for time-division multiplexed signals in an ISDN environment and for digital packets in an ATM environment.
2. Background Information
In North America, the PRI (Primary Rate Interface) version of ISDN (Integrated Services Digital Network) service provides 23 B (Bearer) channels at 64 kbps and a single D channel, also at 64 kbps. In Europe, this version provides 30 B channels and a single D channel. User information is carried on the B channels, with the D channel being used for call control. In North America, this service is provided along a T1 transmission line, while in Europe, this service is provided along an E1 transmission line. The various channels are combined for transmission along a single wire by means of TDM (Time Division Multiplexing), with the channels being given fixed-length frames, or segments of transmission time, in a sequential fashion. Using ISDN, a computing system can place and receive calls over the public switched telephone network to a different client device on each channel, or to another device configured to transmit and receive ISDN data.
Asynchronous Transfer Mode (ATM) service uses a packet-mode technology in which information is transmitted in cells having a fixed length of 53 bytes. Each cell includes a header having a length of 5 bytes, including information needed to send the cell data to the correct location, and a payload of 48 bytes, including the data being transmitted. This ATM technology is used extensively with fiber optical transmission lines to form LANs (Local Area Networks) and backbones tying conventional LANs together. ATM service may also be provided along a T1 data transmission line in North America and along an E1 data transmission line in Europe.
Since many organizations have a need to communicate both with a LAN using ATM technology and with one or more remote locations using ISDN service over the public switched telephone network, what is needed is a communications adapter which can be configured to operate with either or both ATM and ISDN data.